The first person I heard talk about it was Akuzawa Sensei when he was at our dojo. The context was slightly different... He was showing some conditioning exercises and they involved pulling the shoulder blades together. Most of his description centered around developing power by bring everything to the spine.

I was playing with a static technique shortly after his seminar and noticed that when I slid my shoulder blades together (without tightening the shoulder muscles) the partner fell slightly into my space. I started messing with it and realized that this was how you brought the power you received from the partner to your spine. It is in everything!

This is HUGE! It is in everything, or should be. When you combine the shoulder blade element with an understanding of what happens when you tuck your tail bone, your Aikido goes to an entirely different level. The tail bone straightens the spine and allows the pelvis to rotate forward. When this happens, the energy in your arms starts aiming upwards so that when the partner grabs you he is automatically being taken up off his base. This all comes from the core without the arms tensing at all.

Hooker Sensei loved sanchin no kata, which for years I never got. Ushiro Sensei does too, I understand. The same mechanics illustrated in sanchin are in our aikido.

Basically, shoulder blades together straightens the upper lordosis in the same way that the tucking tail straightens the lower lordosis. But they are inverse curves, for a reason. and both reduced curvature (tail-tuck/shoulder blades inward) and accentuated curvature (swayback/shoulder dumped forward) are used in both the high and low spine for very specific and different purposes that can combine in different sequences and effects.

Lower spine demonstration first:

Try this, stand feet apart shoulder width and tuck the tail under to its "normal" "pelvic thrust" limit and then let shift back to "neutral" -- the weight distribution moves from midfoot to toe and back again. You will also note that the sense of torque on your feet is tending to rotate them toes out -heels in.

Now, same position, but sway the back from neutral to its "normal" swayed limit and then back to neutral. The weight distribution still moves from midfoot to toes and back again. You will note that the legs are tending to rotate opposite now -- heels out - toes in (i.e. -- sanchin dachi).

The forward movement of weight distribution is seemingly the same for both (irimi) (Remember that Shioda went on about Ki being in the "Big Toe" i.e. -- the ball of foot. This is what he was referring to.). However, the vertical plane moments and lateral plane (turning torque) moments are exactly reversed .

In tucking tail the moment is forward (irimi) but in an upward arc (aiki-age) or the bottom half of the circle. In swayback, the moment is also forward but arcing down (aiki-sage) or the top half of the circle. The former you use to detach structure from its support -- the latter to pose a teetering structure in a disadvantageously fixed spot.

Lastly, favoring one side or the other in weight distribution then, by itself (review the lower spine illustration and the torque on the feet) develops both vertical plane shear of different signs AND introduces the turning torque moment from the control of the spinal shear alone.

In other words, what Saotome always taught us as a unitary concept -- irimi-tenkan.

Of course, the sequences are not fixed, and fully developed, both of these basic movements can be played with in sequence and allowing the period of rotational development to reverse aspects of the effects, i.e.-- the initial forward-up arc resolves coming down and back; and the forward-down arc resolves coming up and back. The circles of moment are always continuous.

Line backers use the lower lordosis to directly resist toppling moments from the opposing line. That is essentially an exercise of leverage with the lower spine as a sprung lever, compressing the spinal column and stretching the belly ligaments for an increase in the force couple over that fixed moment arm between the spine and the belly.

Aiki however works off shear, not increased leverage. It shifts the center of the effective moment arm around in the structure in the same way a pencil' s center of rotation can be shifted even though there is a fixed point of contact in the "rubber pencil" "illusion." It is not an illusion -- you are seeing the center of rotation actually shifting and when the center of rotation moves your eye interprets it as bending.

Now add the upper back: The shoulder rolled forward or rolled back modulates the upper spine curvature the same way as we do with the lower back. This is also explicitly demonstrated or trained in sanchin.

Rolling the shoulders forward increases upper spine curvature and rolling back decreases curvature. Rolling the shoulders back or forward adds a further component of moment therefore to that initially generated in the lower spine. If they are used in simultaneous opposition, instead of in progression sequencing, they can help stabilize the neutral structure from different applied external moments (pushes).

The increasing curvature of the spine places shear oriented out the belly and decreasing it puts shear out the back, and at a neutral position there is no shear developed. While we displace the hips to generate the initial shear when delivering it we must not have ANY shear anywhere ( especially at the spine) when this is delivered into the target. The shear must ALL be transmitted through the body to the point of application. This is the origin of the one inch punch.

If the body is disposed to transmit it efficiently this generates a cyclic shear wave through the limbs ( like the pencil) that (if done in the correctly resonant rhythm (seen in furitama, tekubi furi) it affects the opponent's structure in ways that are at once mechanically powerful and neuromuscularly sneaky by tricking stabilizing monosynaptic reflex arcs to fire at the wrong point in the balance sway (and which are faster than the voluntary suppression pathways that are trying to recover). The dead giveaway for this "trick" action is that "head snap" seen in demonstrations where the torso moves reflexively forward from a snatch on the arm while the head bobs or snaps backward. It is the inverse of the action of the one inch punch, actually.

As most are aware, when uke grab both wrists one can either drop him or take him off his base. For the maximum forward-down shear, the lower back is swayed and then the shoulders are rolled forward to throw that cyclic shear out through the arms to generate aiki-sage -- crumpling uke in front of you. Reversing this, the lower spine is tucked in and then the upper spine is straightened and the forward-up shear is thrown out the arms resulting in aiki-age, snatching uke off his base. I cannot emphasize enough the sense of weight thrown down the length of the arms, into uke's body through his arms when I do these things.

The point of all this analysis is not to teach newcomers as such but to improve the critical eye of instructors and senior students for correction of themselves and others according to sound mechanics of proven effectiveness in the art. It has improved mine, and made it easier to help others improve because I know what I am seeing happen or not happen.